The invention generally relates to photovoltaic devices. More particularly, the invention relates to photovoltaic devices that include a window layer comprising a high-diffusivity layer and a low-diffusivity layer.
Thin film solar cells or photovoltaic devices typically include a plurality of semiconductor layers disposed on a transparent substrate, wherein one layer serves as a window layer and a second layer serves as an absorber layer. The window layer allows the penetration of solar radiation to the absorber layer, where the optical energy is converted to usable electrical energy. Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaic cells are one such example of thin film solar cells.
During the device fabrication process, the photovoltaic devices (such as, CdTe/CdS cells) are typically exposed to high temperatures, which may result in bulk and grain-boundary inter-diffusion of sulfur or tellurium between the window and absorber layers. Diffusion of sulfur or tellurium between the two layers is desirable as the inter-diffusion results in passivation of defects at the interface and grain boundaries and may also lead to reduction in lattice mismatch, resulting in improved device performance and long-term stability. However, diffusion of sulfur from a thin (<200 nm) window layer to the absorber layer or diffusion of tellurium from the absorber layer to the window layer may be inhomogeneous, resulting in a discontinuous window layer and incomplete interface and grain-boundary passivation. Further, diffusion of tellurium into the window layer may be desirable only at the interface (for example, for better lattice matching) as diffusion of tellurium into the bulk of the window layer may lead to reduction in bandgap or increased absorption in the window layer. A thicker window layer may be desirable for improved sulfur or tellurium diffusion. However, a thicker window layer may lead to low device current density because of greater light absorption in the short wavelength range
Thus, improved photovoltaic devices having window layer configurations that allow for enhanced sulfur or tellurium diffusion into the window or absorber layers, while providing the desired current density, may be desirable. Further, there is a need for improved photovoltaic devices including window layer configurations that provide for improved interface between the window and the absorber layers, resulting in improved device performance and long term stability of the devices.